Drilling an oil well is typically done with a drill bit attached to a long string of drill pipe, called the drill string. The drill string consists of individual sections of drill pipe, each about 40 ft in length with screw joints at each end. Depending on the geological situation, a drill string can be tens of thousand feet long. The drilling torque is usually transmitted from the drilling rig to the bit through the drill string, whereas the axial thrust force or “weight on bit” is provided by the weight of the drill string itself. In modern drilling operations, a set of highly sophisticated tools that may include formation measurement and bit steering systems are inserted between the drill bit and the drill string at the bottom of the well. One embodiment of this tool assembly is called a BHA or “Bottom Hole Assembly”. The steering or measurement systems are housed inside steel pipe in order to protect them against the high downhole hydrostatic pressures and temperatures.
The electronic circuits, actuators, and telemetry systems inside the BHA require electric power to operate. In many cases, electricity is provided by turbine generators that are placed in the drilling mud stream that flows downhole through the inside of the drill pipe. In cases where this is not practical or when continuous power is needed even during mud pump interruptions, batteries are used. While mud turbines can provide a large amount of electrical power, they have high maintenance cost. The use of batteries is also quite expensive due to the amount of space they occupy, along with the associated high costs of environmentally friendly disposal.
Running power cables along the drill string has proven to be impractical and problematic. Prior art relating to these efforts is disclosed in U.S. Pat. No. 4,126,848 to Denison, “Drill String Telemeter System”; U.S. Pat. No. 3,957,118 to Barry et al: “Cable System for use in a Pipe String and Method for Installing and Using the same”; and U.S. Pat. No. 3,807,502 to Heilhecker et al., “Method for Installing an Electric Conductor in a Drill String”; and the publication “Four Different Systems Used for MWD”, W. J. McDonald, The Oil and Gas Journal, pp 115-124, 3 Apr. 1978. Such systems are believed to have suffered from poor reliability and high cost due to of the large number of electrical connectors.
IFP, an international research and development company located in France, developed a system known as “Simphor” which used wireline cables and large, robust wet connectors. It has never been commercialized for measurement while drilling applications. This system is believed to have suffered from interference with the drilling process.
The use of low loss inductive couplers for use in wired pipe strings is known. U.S. Pat. No. 6,866,306, to Boyle et al, describes the use and basic operation of inductive couplers mounted at the sealing faces of drill pipes. However, some downhole oilfield drilling assemblies contain components that preclude the running of electrical wires along the length of the downhole drilling assembly. Such components present an effective barrier to transmission over wires of electrical power and signals. This barrier makes it extremely difficult to provide electric power from surface equipment to instrumentation at lower locations on the drilling assembly, and also makes it impossible for instrumentation at lower locations on a drilling assembly to transmit electrical signals by wired connection up though the drilling assembly.
An article appearing in the Journal of Dynamic Systems, Measurement and Control, June 2004, Vol. 126, entitled “A Self-Energized Sensor for Wireless Injection Mold Cavity Pressure Measurement: Design and Evaluation”, describes a self-energized sensor system for pressure measurement in the injection mold cavity using ultrasound as the information carrier.
During the drilling process of an oil well, the BHA and the drill string experience a great deal of vibrations which stem from the drill bit biting into the formation as well as the dynamics of the drill pipe rotating in the bore hole (axial and lateral vibrations, whirl, stick-slip and torsional vibrations, etc.). The energy contained in these vibrations could be extracted and used to power downhole systems, for example sensors (vibration, temperature, pressure etc.), and telemetry. If an energy harvesting device is used to run a sensing system, power will be needed for the sensor(s), the signal processing electronics and either memory storage or data transmission.
Additionally, knowledge of the resonant characteristics of an energy harvesting device can be used to gather information about the very vibrations (e.g., frequency and amplitude) that the power is harvested from. Since such a device is typically tuned to a certain frequency, the rate of power generation may be used to conclude the frequency of a vibration itself. Simply spoken, only if shaken at its natural frequency, the device will turn on. If it does, some simple electronics may come alive and transmit a standard signal, so that a receiver will be able to notice that this frequency occurred. In combination with prior modal analysis of the BHA, the devices may be tuned to certain “critical” frequencies, and issue a warning signal if those frequencies occur.
In view of the above, a system, apparatus and method for exploiting the inherent vibration present near a drill bit in a drilling assembly for use in power and signal generation is necessary.